The present invention relates to a cathode active material containing a lithium composite oxide and a non-aqueous electrolyte secondary battery using the cathode active material in a positive electrode.
In recent years, as the semiconductor integration technique is advanced, various electronic devices, such as portable phones and laptop computers, are being reduced in size and weight. For this reason, as a portable power source for these electronic devices, there is desired a high performance secondary battery which is small in size and lightweight and which has a long life. As examples of secondary batteries meeting the demands, there can be mentioned nickel-hydrogen batteries, nickel-cadmium batteries, and lithium ion secondary batteries, and especially lithium ion secondary batteries have a voltage and an energy density as high as those of a 4-V grade battery, and hence can be mounted on an electronic device causing large power consumption. Thus, the lithium ion secondary batteries are mounted on portable electronic devices which primary batteries cannot deal with.
A feature of the lithium ion secondary battery resides in that a positive electrode having a high oxidation-reduction potential and a negative electrode having a low oxidation-reduction potential are combined to achieve a large capacity, i.e., large energy density, as compared to those of other batteries. Many electronic devices are actually used not only in an environment at room temperature but also in environments at temperatures in a wide range of from a low temperature to a high temperature, and the energy density varies depending on the way of using the device. In addition, when discharging at a large current is conducted, the quantity of electricity which can be taken out is reduced, and the voltage is also lowered due to the internal resistance.
In the lithium ion secondary battery, as a cathode active material, a lithium-cobalt composite oxide is mainly used (see, for example, Japanese Patent Application Publication No. 2003-242976 and Japanese Patent Application Publication No. 2001-319652). The reason for this is that the lithium-cobalt composite oxide has advantages in that the average discharge potential is high, and the like.
However, the lithium-cobalt composite oxide has problems in that the cycle characteristics are poor, and that the voltage is markedly lowered when discharging at a large output is conducted at a low temperature, for example, when a personal computer is used in a cold district, the computer cannot be started up due to a lack of the output of the power source.
Recently, as the application of the lithium ion battery is expanded, not only the further improvement of the cycle characteristics but also the increase of the output at low temperatures are demanded, and therefore a non-aqueous electrolyte battery excellent in both cycle characteristics and low-temperature load characteristics is desired.
It is desirable to provide a cathode active material and a non-aqueous electrolyte secondary battery, which are excellent in both cycle characteristics and low-temperature load characteristics.